1. Field of the Invention
The present invention relates to an optical fiber manufacturing apparatus and method, and in particular, an optical fiber manufacturing apparatus and method suitable for stable drawing of a large optical fiber preform when manufacturing a bare optical fiber by drawing of an optical fiber preform.
2. Description of the Related Art
In general, as an apparatus for manufacturing a silica glass based optical fiber, an apparatus shown in FIG. 5 is widely used. An optical fiber manufacturing apparatus 10 is configured to include a drawing furnace 14, a cooling device 18, a resin coating device (coating device) 20, a curing device 22, a take-up capstan 26, and a winding device 28. The drawing furnace 14 includes a heater 14A for heating and melting an optical fiber preform 12. The cooling device 18 forcibly cools a bare optical fiber 16 that is linearly pulled out downward from the drawing furnace 14. The resin coating device (coating device) 20 coats the cooled bare optical fiber 16 with a resin for protective coating. The curing device 22 cures the resin coated by the resin coating device 20, and is configured to include an ultraviolet (UV) irradiation device, a thermal cross-linking device, or the like. The take-up capstan 26 takes up an optical fiber 24 in a state in which the resin for protective coating has been cured.
When manufacturing the optical fiber using such an optical fiber manufacturing apparatus, first, the optical fiber preform (silica based glass preform) 12 that is a source material of the bare optical fiber is melted by being heated to the temperature of 2000° C. or higher in the drawing furnace 14. Then, the optical fiber preform 12 is pulled out (drawn) downward in a thin line shape from the bottom of the drawing furnace 14 while extending as the bare optical fiber 16 in the high-temperature state. Then, the bare optical fiber 16 is cooled by the cooling device 18 to a temperature at which the bare optical fiber 16 can be coated with resin. Then, the bare optical fiber 16 that has been cooled to the required temperature is coated (applied) with the resin for protection in an uncured state by the resin coating device 20. Then, the coated resin is cured by UV curing or heat curing in the curing device 22, and the optical fiber 24 having a protective coating layer is manufactured. Then, the optical fiber 24 is wound by the winding device 28 through the take-up capstan 26.
Incidentally, in the manufacturing of this type of optical fiber, the size of the optical fiber preform and the drawing speed have increased in order to improve the productivity. In addition, various problems have occurred with an increase in the drawing speed. For example, a problem of sealing (airtightness) between the optical fiber preform and the heating furnace, a problem of alignment of the bare optical fiber 16 pulled out from the heating furnace 14 with respect to the die hole of the resin coating device 20, and the like are likely to occur, and the solutions have been demanded.
That is, in the drawing furnace, in order to prevent the oxidation of carbon components or the like in the heating furnace, inert gas, such as argon gas, is supplied to the heating furnace. In this case, stability in the sealing (airtightness) of the space around the optical fiber preform in the heating furnace, especially, sealing between the outer peripheral surface of the optical fiber preform and the upper opening of the heating furnace, has been demanded. That is, if a gap between the outer surface of the optical fiber preform and the upper opening end of the heating furnace is not uniform, the flow of gas in the heating furnace is uneven. Accordingly, the outer diameter variation of the bare optical fiber is increased or the optical fiber preform heating temperature is not uniform. As a result, there is a possibility that the cross-sectional shape of the bare optical fiber pulled out from the heating furnace will deviate largely from the true circle (will become non-circular). In addition, with an increase in the size of the optical fiber preform, the outer diameter variation of the optical fiber preform has also increased, and bending (deflection) is also likely to occur in the optical fiber preform. For this reason, it is difficult to stabilize the sealing performance (airtightness) by maintaining uniformity of the gap between the outer surface of the optical fiber preform and the upper opening end of the heating furnace.
In addition, in the manufacturing of the optical fiber, it is necessary to precisely match the position of the bare optical fiber pulled out vertically downward from the heating furnace with the die center position in the resin coating device. That is, if the bare optical fiber pulled out from the heating furnace does not pass through the die center in the resin coating device, coating of the resin with respect to the bare optical fiber is not uniform, and there is a problem that a thickness of the coating layer is likely to become uneven. Therefore, it is necessary to match the position of the bare optical fiber with the die center position in the resin coating device, and this is generally referred to as alignment. However, since the bending (deflection) of the optical fiber preform also increases with an increase in the size of the optical fiber preform, the alignment also becomes difficult.
In addition, stabilization of the sealing performance of the heating furnace and appropriate control of the alignment become still more difficult with an increase in the drawing speed.
Incidentally, in Japanese Unexamined Patent Application, First Publication No. H3-37128 (hereinafter referred to as “PTL 1”), as shown in FIG. 6, a cylindrical shield pipe 30 connected to a heating space 15 in a heating furnace is provided above the heating furnace 14. The shield pipe 30 stabilizes the sealing (airtightness of the drawing furnace 14) between the inner space (heating space) 15 of the drawing furnace 14 and the optical fiber preform 12. In addition, PTL 1 discloses an optical fiber drawing furnace (drawing furnace) including a sending rod 32, a sealing member 34, and a connecting member 36. The sending rod 32 is provided in the shield pipe 30, and lowers according to the amount of drawing of the optical fiber preform 12. The sealing member 34 is provided around the lower end of the sending rod 32, and fits in the shield pipe 30 so as to be vertically movable. The connecting member 36 is provided at the lower end of the sending rod 32, and connects the lower end to the upper end of the optical fiber preform 12.
In PTL 1, the sealing performance of the heating furnace 14 is ensured between the shield pipe 30 and the sealing member 34 in the shield pipe. Therefore, PTL 1 discloses that good drawing is possible since it is possible to maintain the stable sealing performance without dependence on the diameter of the optical fiber preform.
Japanese Unexamined Patent Application, First Publication No. S60-137842 (hereinafter referred to as “PTL 2”) discloses that an adjusting device (alignment control device) configured to adjust the position of the optical fiber preform in X and Y directions within the horizontal plane is provided on the upper side of the heating furnace. Specifically, PTL 2 discloses that the position of the preform is controlled by detecting the position of the bare fiber, which is pulled out vertically downward from the heating furnace, immediately below the heating furnace and inputting the detected signal to the alignment control device. In addition, if there is misalignment of a gripping in a chuck for gripping the optical fiber preform, bending of the preform itself, or the like, the clearance between the preform and an airtight plate (upper airtight plate) provided at the upper opening end of the drawing furnace may become asymmetrical or the preform and the airtight plate may be in contact with each other. Therefore, PTL 2 discloses that the position of the upper airtight plate is also controlled to keep the above mentioned clearance constant.
In addition, Japanese Unexamined Patent Application, First Publication No. H4-130030 (hereinafter referred to as “PTL 3”) discloses that the position of the bare optical fiber immediately below the drawing furnace is detected and the position of the optical fiber preform is adjusted based on the detection signal similar to PTL 2. Here, as a preform position adjusting method, not only moving a holder portion configured to grip the preform but also fixing the drawing furnace and the preform holder to a single movable table, making the table slidable within the horizontal plane, detecting the position of the bare optical fiber using a fiber position detecting device immediately below the movable table, and making the movable table slide based on the position detection signal is disclosed. In addition, PTL 3 discloses that an alignment device is interposed between the movable table and the support base in such a manner described above so that other mechanisms are neither enlarged nor complicated.
The overview of the related arts disclosed in the above PTLs is as follows.
PTL 1: In order to stabilize the sealing performance of the drawing furnace, a shield pipe is connected to the upper portion of the heating furnace configured to seal between the inner surface of the shield pipe and the outer surface of the sealing member around the sending rod.
PTL 2: Both a holder portion configured to support the optical fiber preform and the upper airtight plate of the drawing furnace are aligned.
PTL 3: In order to align the bare optical fiber position, the drawing furnace and the preform holder are placed on the same table, and alignment is performed for each table.
The above techniques disclosed in PTLs 1 to 3 have the following problems.
That is, in the case of PTL 1, there is no alignment mechanism. Therefore, it is not possible to respond to deflection or bending when the optical fiber preform is large.
In the case of PTL 2, since there is no shield pipe, it is difficult to ensure the sealing performance when using a preform with an outer diameter variation in the longitudinal direction of the optical fiber preform. In addition, it is necessary to use an airtight plate that matches the outer diameter of each preform, but replacement of the airtight plate is complicated when optical fiber preforms of various diameters are targets.
In addition, in the case of PTL 3, misalignment between the drawing furnace and the optical fiber preform is not resolved. For this reason, when the preform is bent or when the preform setting position deviates from the center of the drawing furnace, the distribution of the heat applied to the preform becomes non-uniform in the circumferential direction. Thus, since the roundness of the cross section of the bare optical fiber becomes worse or the flow of gas within the drawing furnace becomes uneven, the outer diameter variation of the bare optical fiber is increased.
Therefore, in the related arts proposed, it has been difficult to always effectively ensure stable sealing performance for the bending or deflection of the optical fiber preform, the outer diameter variation of the optical fiber preform in the longitudinal direction, or the outer diameter variation for each lot.
In addition, although the techniques of PTLs 1 to 3 may be combined in some cases, there are the following problems even if these are combined.
For example, when the technique of PTL 1 and the technique of PTL 2 are combined, it is necessary to align the shield pipe itself. In this case, not only does a problem in the movable structure and the method for sealing between the upper portion of the drawing furnace and the connecting portion (contact portion) of the shield pipe occurs, but also the alignment device becomes large and complicated.
In addition, when the technique of PTL 1 and the technique of PTL 3 are combined, it is possible to move the drawing furnace, the sending rod, and the shield pipe in a state where all of the drawing furnace, the sending rod, and the shield pipe are placed on the same movable table, but the problem of misalignment between the drawing furnace and the optical fiber preform still remains unresolved as described above.
The present invention has been made in view of the above situation, and it is an object of the present invention to provide an optical fiber manufacturing apparatus that includes a sealing member, which can effectively ensure the always stable sealing performance for the bending or deflection of the optical fiber preform, the outer diameter variation of the optical fiber preform in the longitudinal direction, or the outer diameter variation for each lot, and that is not enlarged and complicated, and an optical fiber manufacturing method using the optical fiber manufacturing apparatus.